Synthesis of nano-Na3V2(PO4)2F3 cathodes with excess Na+ intercalation for enhanced capacity

Vanadium-based materials are promising as cathodes for sodium-ion batteries owing to their superior cycling behavior and rate performance. However, there is a critical need for the rational implementation of Na+ intercalation to enhance the capacity. Herein, nano-Na3V2(PO4)2F3 materials were synthesized via bulk dismemberment by gas released by the decomposition of excess citric acid to trigger some degree of lattice distortion and defects. Owing to the weakened electrostatic repulsion between sodium ions in Na3V2(PO4)2F3 with an enlarged interlayer spacing, one more Na+ ion could be readily embedded to form Na4V2(PO4)2F3. A new reaction plateau at 1.38 V/1.56 V accompanied the V3+/V2+ redox reaction, inducing a capacity of 250 mA h·g−1, which is higher than that of highly crystallined Na3V2(PO4)2F3. A noteworthy reversible redox phenomenon involving three sodium ions was confirmed during the cycling process. The nano-Na3V2(PO4)2F3 electrode exhibited a capacity retention of 72% at 1.3–4.5 V after 20 cycles. Thus, this novel synthesis routine can provide vanadium-based cathode materials with enhanced Na+ intercalation properties, which can be applied to improve the capacity of sodium-ion batteries.